FIGURE 24.2 Roof framing for a Circular Dome.

The same concept can be observed in the design of a circular dome. Again, there are two different ways of framing a dome. The dome shown in Figure 24.2a is a complex of elements like arches, primary and secondary beams, and purlins, which all lie in a plane. Each of these elements constitutes a system that is stable by itself. In contrast, the dome shown in Figure 24.2b is an assembly of a series of longitudinal, meridional, and diagonal members, a form of latticed shell. It is a system whose resisting capacity is ensured only through its integral action as a whole.

The difference between planar structures and space frames can be understood also by examining the sequence of flow of forces. In a planar system, the force due to the roof load is transferred successively through the secondary elements, the primary elements, and then finally to the foundation. In each case, loads are transferred from the elements of lighter class to the elements of heavier class. As the sequence proceeds, the magnitude of the load to be transferred increases, so does the span of the element. Thus, elements in a planar structure are characterized by their distinctive ranks, not only by the size of their cross-sections but also by the importance of the task assigned to them. In contrast, in a space system there is no sequence of load transfer, and all elements contribute to the task of resisting the roof load in accordance with the three-dimensional geometry of the structure. For this reason, the ranking of the constituent elements similar to planar structures is not observed in a space frame.